ARTICLE Received 19 Dec 2012 | Accepted 26 Feb 2013 | Published 16 Apr 2013 DOI: 10.1038/ncomms2664 Intercalation and delamination of layered carbides and carbonitrides Olha Mashtalir1,2, Michael Naguib1,2, Vadym N. Mochalin1,2, Yohan Dall’Agnese1,2, Min Heon1,2, Michel W. Barsoum1 & Yury Gogotsi1,2 Intercalation and delamination of two-dimensional solids in many cases is a requisite step for exploiting their unique properties. Herein we report on the intercalation of two-dimensional Ti3C2,Ti3CN and TiNbC—so called MXenes. Intercalation of hydrazine, and its co-inter- calation with N,N-dimethylformamide, resulted in increases of the c-lattice parameters of surface functionalized f-Ti3C2, from 19.5 to 25.48 and 26.8 Å, respectively. Urea is also intercalated into f-Ti3C2. Molecular dynamics simulations suggest that a hydrazine monolayer intercalates between f-Ti3C2 layers. Hydrazine is also intercalated into f-Ti3CN and f-TiNbC. When dimethyl sulphoxide is intercalated into f-Ti3C2, followed by sonication in water, the f-Ti3C2 is delaminated forming a stable colloidal solution that is in turn filtered to produce MXene ‘paper’. The latter shows excellent Li-ion capacity at extremely high charging rates. 1 Department of Materials Science and Engineering, Drexel University, 3141 Chestnut Street, Philadelphia, Pennsylvania 19104, USA. 2 A.J. Drexel Nanotechnology Institute, Drexel University, 3141 Chestnut Street, Philadelphia, Pennsylvania 19104, USA. Correspondence and requests for materials should be addressed to Y.G. (email: [email protected]). NATURE COMMUNICATIONS | 4:1716 | DOI: 10.1038/ncomms2664 | www.nature.com/naturecommunications 1 & 2013 Macmillan Publishers Limited. All rights reserved. ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms2664 unique characteristic of layered materials is their ability DMF at 80 °C for 24 h, the c-LPs increased to 25.48±0.02 and to accommodate various ions and molecules between 26.8±0.1 Å, respectively. The larger c-LP increase in the latter Atheir layers, a phenomenon known as intercalation. case points to a synergistic effect when HM is dissolved in DMF Intercalation compounds of inorganic hosts (graphite, clays, before intercalation. di-chalcogenides and others) have also gained renewed interest When the HM-intercalated powders were heated to 120 °Cina owing to their unique chemical and physical properties1–6. vacuum oven, the c-LP decreased from 25.48±0.02 to De-intercalation of those compounds under certain conditions 20.6±0.3 Å, signifying that the intercalation process is essentially (such as thermal shock) as well as sonication or volume ex- reversible. Heating powders intercalated with HM and DMF pansion reactions in the interlayer space lead to the formation of under the same conditions resulted in small increases in the 2y widely used exfoliated graphite7–11 and two-dimensional (2D) angles (Fig. 1b-ii,c-ii). Thus, the HM/DMF combination is more materials with unusual electronic properties12–15. resistant to de-intercalation than HM alone, possibly because of Recently, a large family of 2D materials, labelled MXenes— the higher boiling point of DMF (153 °C) compared with that of produced by the extraction of the A-element from the layered HM (114 °C). However, when f-Ti3C2 intercalated with HM and 16 ternary carbides, such as Ti3AlC2 and other MAX phases , DMF was vacuum dried at 200 °C, the c-LP decreased to where M is an early transition metal, A is an A group element, 20.1±0.5 Å. and X is C or N—was discovered17,18. The MXenes produced to X-ray photoelectron spectroscopy (XPS) (Supplementary Fig. S2) 17 date, viz., Ti3C2 (ref. 17), Ti2C, Ta4C3, (Ti0.5Nb0.5)2C, provides further evidence of intercalation. As previously reported , (V0.5Cr0.5)3C2 and Ti3CN (ref. 18) are, in many aspects, exfoliated f-Ti3C2 showed the presence of Ti–C and Ti–O bonds, as graphene-like, forming stacks of sheets and scrolls. They also well as OH groups, suggested by the presence of a O1s peak around are good electrical conductors17–21 and are predicted to have high 532 eV (see Supplementary Materials for more details). A N1s 22 elastic moduli. Electrochemical intercalation of Li ions between signal was also observed around 400 eV in XPS spectra of f-Ti3C2 the MXene sheets renders these solids promising materials for Li- treated with HM and HM/DMF (insets in Supplementary ion battery anodes and hybrid electrochemical capacitors23–25. Figs S2a,b, respectively). No nitrogen peaks were detected in However, up to now there have been no reports of the chemical f-Ti3C2 before intercalation. intercalation or large-scale delamination of MXenes or any other Scanning electron microscopy (SEM) images of f-Ti3C2, before transition metal carbides. and after HM treatment in DMF at 80 °C for 24 h, shown in It is important to note at the outset that the 2D MXene surfaces Fig. 2a,b, respectively, confirm that: (i) the MXene remains are not M-terminated, but covered with oxygen-containing exfoliated after intercalation, and, (ii) the multilayers thicken groups, such as OH, and fluorine (F) introduced after etching (Fig. 2b) apparently by gluing individual monolayers together, with aqueous hydrofluoric acid (HF)17. The chemistry of forming 20–50-nm-thick lamellas. Note that the absence of XRD exfoliated MXene produced by HF etching of Ti3AlC2 is much peaks corresponding to a c-LP of 19.5 Å (Fig. 1b-i,c-i) implies that closer to Ti3C2(OH)xOyFz than to the idealized structure of a pure intercalation was essentially complete. carbide layer Ti3C2. Henceforth, and for brevity’s sake, f-Ti3C2 Transmission electron microscopy (TEM) images and will be used instead of the more cumbersome Ti3C2(OH)xOyFz. corresponding selected area electron diffraction patterns of The original aim of this work was to reduce the f-Ti3C2 surfaces f-Ti3C2 intercalated with HM in DMF at 80 °C for 24 h to create Ti-terminated surfaces that theory predicts would be (Fig. 2e,f) showed that the basal planes’ hexagonal structure magnetic25. A perusal of the graphene literature made it clear that remains unchanged (compare inset in Fig. 2d with Fig. 2f) after 17 hydrazine monohydrate N2H4 Á H2O (HM) dissolved in N,N- intercalation . The interplanar spacing in intercalated f-Ti3C2— dimethylformamide (DMF) is the reactant of choice26. Since in 2.648 Å for (0110) and 1.540 Å for (0210)—results in an a-LP of our case, HM was found to primarily act as an intercalant rather 3.057 Å, in excellent agreement with the a-LP of f-Ti3C2 before 17 than a reducing agent, the research objectives were redirected intercalation and that of the Ti3AlC2, viz., 3.058 Å . towards intercalation and delamination of MXenes. Furthermore, as To demonstrate that intercalation is a general phenomenon the layered structures of the MXenes retain some similarities to rather than the exclusive property of f-Ti3C2, the MXenes Ti3CN 17 clays , we reviewed the intercalation chemistries of the latter. and TiNbC were also treated with HM. Similar to f-Ti3C2, the shift Numerous compounds, such as formamide and its derivatives, of the major XRD peak of these MXenes to lower 2y values dimethyl sulphoxide (DMSO), urea and long-chain alkylamines, (Supplementary Fig. S3) confirmed their intercalation. It is thus among others have been shown to intercalate clays27.Forclays,HM reasonable to conclude that f-MXenes—like other layered is probably the most common intercalant; its intercalation results in materials—can be readily intercalated by many different the increase of kaolinite’s c-lattice parameter (c-LP) from 7.2 to compounds. 10.3–10.4 Å (refs 27–29). In addition, the electrical resistivities of non-intercalated MXenes Here we report, for the first time, on the intercalation of and MXenes treated with HM were measured. The resistivity values f-Ti3C2 with urea, HM, HM dissolved in DMF and DMSO. of all intercalated samples were higher than those of non- Importantly, intercalation of DMSO enabled us to delaminate intercalated MXenes presumably owing to the increase of their c- stacked f-Ti3C2 layers into separate 2D MXene sheets. We also LPs after intercalation (Supplementary Table S1). The differences in show that HM intercalates Ti3CN and TiNbC. the magnitudes of the resistivity increases for different intercalated MXenes, at relatively the same expansion, can be partially explained by the different number of MXene atomic layers. In case of M3X2 Results (five atomic layers), the resistivity increases by an order of Intercalation of MXenes. A schematic of MXene intercalation is magnitude, whereas an increase by two orders is observed for the shown in Fig. 1a. X-ray diffraction (XRD) showed that after M2C (three atomic layers) compounds. intercalation with HM and/or DMF, the (000l) peaks were still present, but shifted to lower 2y angles (Fig. 1b,c and Supplementary Fig. S1). Table 1 summarizes the c-LPs values for Molecular dynamics (MD) simulations. To further our f-Ti3C2 treated with HM and HM in DMF. The initial c-LP was understanding of the structure of intercalated MXene, MD 19.5±0.1 Å, a value that does not change much with post- simulations of N2H4 inserted between OH-terminated Ti3C2 intercalation drying (Table 1). After exposure to HM or HM in layers were carried out (Fig. 3a). MXene supercells (4 Â 2 Â 1 2 NATURE COMMUNICATIONS | 4:1716 | DOI: 10.1038/ncomms2664 | www.nature.com/naturecommunications & 2013 Macmillan Publishers Limited. All rights reserved. NATURE COMMUNICATIONS | DOI: 10.1038/ncomms2664 ARTICLE a MAX MXene Intercalated MXene c Al removal Intercalation bcRT RT 120 °C 120 °C iii iii 120 °C 120 °C RT C RT 200 °C 200 °C Ti ii ii Al Intensity (arb.u.) Intensity (arb.u.) O N i i H 5 10152025303540 5 6 7 8 9 10 11 12 2θ (deg) 2θ (deg) Figure 1 | Evidence of MXene intercalation.